implement alpha locking per layer

engine/shader.h

@@ -9,6 +9,7 @@ enum class kShaderUniform : uint16_t
     Tex = const_hash("tex"),
     TexFG = const_hash("tex_fg"),
     TexBG = const_hash("tex_bg"),
+    TexMask = const_hash("tex_mask"),
     Col = const_hash("col"),
     Tof = const_hash("tof"),
     Tsz = const_hash("tsz"),
@@ -27,10 +28,11 @@ enum class kShader : uint16_t
     Font        = const_hash("font"),
     Atlas       = const_hash("atlas"),
     Stroke      = const_hash("stroke"),
+    StrokeLock = const_hash("stroke-lock"),
     StrokeErase = const_hash("stroke-erase"),
     StrokeLayer = const_hash("stroke-layer"),
     Checkerboard= const_hash("checkerboard"),
-    Equirect    = const_hash("equirect"),
+    Equirect = const_hash("equirect"),
 };
 
 class Shader